1. Field of the Invention
The present invention relates to a tracking error signal generation device, an optical disc apparatus, a tracking error signal generation method, and a tracking control method.
2. Description of the Related Art
It has been conventionally known to use a tracking error signal generation device for generating a tracking error signal in an optical disc apparatus for recording information on or reproducing information from an optical disc. A tracking error signal is a signal for indicating whether or not a beam is moving along a track provided in an optical disc.
A conventional tracking error signal generation device generates a tracking error signal as follows.
An optical beam emitted by a laser is split into a main beam, a first sub beam and a second sub beam by diffraction grating. The main beam, the first sub beam and the second sub beam are converged onto an optical disc by an objective lens. The main beam, the first sub beam and the second sub beam are reflected by the optical disc, and the reflected main beam, first sub beam and second sub beam are detected by a light detector.
In more detail, the light detector includes a two-portion main beam detector, a two-portion first sub beam detector, and a two-portion second sub beam detector. The two-portion main beam detector detects the main beam and generates a differential signal. The two-portion first sub beam detector detects the first sub beam, and generates a differential signal. The two-portion second sub beam detector detects the second sub beam and generates a differential signal.
A main beam push-pull signal generator generates a main beam push-pull signal based on the differential signal generated by the two-portion main beam detector.
A sub beam push-pull signal generator generates a sub beam push-pull signal based on the differential signal generated by the two-portion first sub beam detector and the differential signal generated by the two-portion second sub beam detector. In more detail, the sub beam push-pull signal generator generates a sub beam push-pull signal by adding the differential signal generated by the two-portion first sub beam detector and the differential signal generated by the two-portion second sub beam detector.
Next, the sub beam push-pull signal is amplified at an appropriate gain ratio as necessary, and the amplified sub beam push-pull signal is subtracted from the main beam push-pull signal. Thus, a tracking error signal is generated.
The conventional tracking error signal generation device generates a tracking error signal in this manner. Accordingly, even if the objective lens is displaced and thus the main beam push-pull signal and the sub beam push-pull signal each obtain a DC offset, the DC offset is deleted from the tracking error signal by subtracting the sub beam push-pull signal from the main beam push-pull signal as disclosed in, for example, Japanese Laid-Open Publication No. 61-94246.
However, the amplitude of the tracking error signal may undesirably fluctuate.
For example, when an optical disc is eccentrically provided, the amplitude of the tracking error signal is reduced.
When the objective lens is displaced, the main beam push-pull signal is also displaced from the reference value.
Hereinafter, the case where the optical disc is eccentrically provided will be described.
An optical disc, which is rotated by a spindle motor, has a hole at the center thereof to which the spindle motor is attached. The hole is provided such that the center of the hole matches the center of the optical disc, namely, the center of a plurality of tracks formed spirally or concentrically.
However, the center of the optical disc may be undesirably deviated from the center of the hole, in which case the optical disc is rotated eccentrically.
Hereinafter, a tracking error signal when the optical disc is rotated normally, i.e., not eccentrically, will be described with reference to FIGS. 9A and 9B. Then, a tracking error signal when the optical disc is rotated eccentrically will be described with reference to FIGS. 10A and 10B.
FIG. 9A shows a schematic view illustrating the relationship between the tracks and the scanning direction of a main beam M, a first sub beam S1 and a second sub beam S2 when the optical disc is rotated normally.
The optical disc has tracks concentrically or spirally formed. The tracks are provided such that adjacent tracks are parallel to each other.
As shown in FIG. 9A, the distance between the center of the optical spot formed of the first sub beam S1 and the center of the optical spot formed of the main beam M is ½ of the track pitch. Similarly, the distance between the optical spot formed of the center of the second sub beam S2 and the center of the optical spot formed of the main beam M is ½ of the track pitch.
The first sub beam S1 is directed to a position at the center between the track irradiated by the main beam M and a track which is adjacent and outer to the track.
The second sub beam S2 is directed to a position at the center between the track irradiated by the main beam M and a track which is adjacent and inner to the track.
A center line 900 represents the direction in which the main beam M scans the optical disc. In FIG. 9A, the center line 900 and the tracks are parallel to each other.
FIG. 9B is a waveform diagram illustrating the main beam push-pull signal and the sub beam push-pull signal as the main beam M moves along the tracks in the case where the optical disc is rotated normally.
In FIG. 9B, “Mpp” refers both to a main beam push-pull signal based on the main beam M and an amplitude thereof, “Spp1” refers both to a differential signal generated by the two-portion first sub beam detector and an amplitude thereof, and “Spp2” refers both to a differential signal generated by the two-portion second sub beam detector and an amplitude thereof. Spp1+Spp2 is the sub beam push-pull signal. The horizontal axis represents the position of the main beam M.
A tracking error signal TE is obtained by the following expression.TE=Mpp−K×(Spp1+Spp2)where K is a prescribed constant.
When the optical disc is rotated normally, the phase of the differential signal Spp1 generated by the two-portion first sub beam detector is the same as the phase of the differential signal Spp2 generated by the two-portion second sub beam detector. The two signals are not deviated in phase from each other. In this state, the sub beam push-pull signal, i.e., Spp1+Spp2, has an amplitude of E.
The tracking error signal is generated by multiplying the sub beam push-pull signal by the prescribed constant K and then subtracting the multiplication result from the main beam push-pull signal.
Next, with reference to FIGS. 10A and 10B, a tracking error signal when the optical disc is rotated eccentrically will be described.
FIG. 10A shows a schematic view illustrating the relationship between the tracks and the scanning direction of the main beam M, the first sub beam S1 and the second sub beam S2 when the optical disc is rotated eccentrically.
As shown in FIG. 10A, when the optical disc is rotated eccentrically, the first sub beam S1 is deviated from the center between the track irradiated by the main beam M and the track which is adjacent and outer to the track in accordance with the rotating angle of the optical disc. Similarly, the second sub beam S2 is deviated from the center between the track irradiated by the main beam M and the track which is adjacent and inner to the track in accordance with the rotating angle of the optical disc.
In FIG. 10A, a center line 1000 is not parallel to the tracks, and a portion of the first sub beam S1 and a portion of the second sub beam S2 are directed to a track 1001.
FIG. 10B is a waveform diagram illustrating the main beam push-pull signal and the sub beam push-pull signal in the case where the optical disc is rotated eccentrically.
In FIG. 10B, Mpp” refers both to a main beam push-pull signal based on the main beam M and an amplitude thereof, “Spp1” refers both to a differential signal generated by the two-portion first sub beam detector and an amplitude thereof, and “Spp2” refers both to a differential signal generated by the two-portion second sub beam detector and an amplitude thereof. Spp1+Spp2 is the sub beam push-pull signal. The horizontal axis represents the position of the main beam M.
In the case where a portion of the first sub beam S1 and a portion of the second sub beam S2 are directed to a track 1001 as shown in FIG. 10A, the phase of the differential signal Spp1 is deviated with respect to the phase of the differential signal Spp2 as shown in FIG. 10B.
Here also, a tracking error signal TE is obtained by the following expression.TE=Mpp−K×(Spp1+Spp2),where K is a prescribed constant.
When the optical disc is rotated eccentrically, the phase of the differential signal Spp1 is deviated with respect to the phase of the differential signal Spp2. Accordingly, where the amplitude of the sub beam push-pull signal, i.e., Spp1+Spp2, is R, the amplitude R is smaller than the amplitude E of the sub beam push-pull signal described-above with reference to FIG. 9B.
As described above, a tracking error signal is generated by multiplying the sub beam push-pull signal by the prescribed constant K and then subtracting the multiplication result from the main beam push-pull signal. Therefore, when the amplitude of the sub beam push-pull signal is smaller, an appropriate tracking error signal cannot be generated.
When the amplitude of the sub beam push-pull signal is reduced, the open loop gain of the tracking control system is reduced, which makes the tracking control system unstable.
The conventional tracking error signal generation device has the following problem in addition to the problem that the amplitude of the tracking error signal fluctuates.
Recently, it has been desired to increase the memory capacity of optical discs. In order to meet this desire, it has been proposed to provide an optical disc with a plurality of information faces. In such an optical disc, information can be recorded on or reproduced from the plurality of information faces by directing an optical beam from a prescribed direction.
When the information recorded on a prescribed information face is reproduced, focusing control is performed such that the optical beam is converged onto the prescribed information face. Tracking control is performed such that the optical beam moves along the tracks of the prescribed information face.
While the information recorded on the prescribed information face is reproduced, an optical beam passing through the prescribed information face may be undesirably reflected by an information face different from the prescribed information face and is incident on the light detector.
Generally in a tracking error signal generation device, a light amount of a sub beam is about 1/10 of the a light amount of a main beam. A light amount of the sub beam reflected by a prescribed information face is smaller than a light amount of the main beam reflected by the prescribed information face. Therefore, when a main beam passing through the prescribed information face is reflected by an information face different from the prescribed information face and is incident on the light detector, the light amount thereof which is incident on the two-portion first sub beam light detector and the two-portion second sub beam light detector is not negligible.
The light amount of the main beam which is incident on the two-portion first sub beam light detector and the two-portion second sub beam light detector varies in accordance with, for example, the distance between the prescribed information face and the different information face being changed. The distance between a track or portions thereof of the prescribed information face and the corresponding track or portions thereof of the different information face of the optical disc varies. Therefore, while the optical disc is rotating, the light amount of the main beam reflected by the different information face changes by a cycle of several hundred hertz.
A portion of the main beam reflected by the different information face acts as an external disturbance to the sub beam push-pull signal, resulting in the tracking error signal including an offset component. Therefore, the tracking cannot be appropriately controlled, which may undesirably deteriorate the recording and/or reproduction quality.